1. Field of the Invention
The present invention relates to a Sigma-Delta CODEC system, more particularly, providing the function of software-controlled digital signal processor for operational analysis by the processing ability of CPU. The present invention provides various digital signal processes that the Virtual Device Driver (Vxd) mates with the processing ability of CPU without replacing by other hardware device (such as the sound card).
2. Description of the Prior Art
Analog to digital (A/D) conversion and digital to analog (D/A) conversion are used in a general modulation/demodulation device,namely termed as a coder/decoder (CODEC). In the prior art, there are many methods used for analog to digital and digital to analog conversion, for instance, integration, successive approximation, parallel conversion, delta modulation, pulse code modulation (PCM) and Sigma-Delta conversion. The selection of each translation method depends on various situations and needs. For example, systems needing high resolution of audio and video signals, generally use pulse code modulation or Sigma-Delta conversion.
Pulse code modulation (PCM) is characterized by very high oversampling rates, noise shaping, and word lengths of one or a few bits. Nevertheless, pulse code modulation (PCM) can introduce a differential nonlinearity error to the amplitude representation. Furthermore, the PCM may modify after processing, because the noise signal of each bit is unequal to that of each other bit, and therefore in each sampling, the total of each noise signal is unequal to that of the total of each noise signal.
In consideration of this disadvantage of pulse code modulation (PCM), the Sigma-Delta conversion method provides different solution for applying to the field of speech recognition. Such a method belongs to the low-bit conversion type: it carries out sampling and conversion through the use of oversampling. The advantages of this method not only can substantially enhance resolution, but also can create noise shaping. In other words, high resolution is achieved through noise shaping.
FIG. 1 is a block diagram of a prior art Sigma-Delta CODEC system. The traditional method of A/D conversion is to input the analog signal to the modulation/demodulation device 10, by way of the processes of the digital signal processer 20 and the low pass filter 30, to obtain the analog output. The traditional Sigma-Delta CODEC consists of the analog circuit and the digital filter circuit. The traditional design combines the analog circuit with the digital filter circuit on the same main board. Low-bit A/D and D/A converters both use conversion methods such as Sigma-Delta modulation with noise shaping, and both process high sampling-rate signals with oversampling and decimation filters. In other words, hardware techniques used in high-speed digital signal processors must be used to achieve the high performance digital filter necessary to form a quality CODEC. However, this might facilitate miscellaneous circuit density, sophisticated circuit design, and high production cost.